This invention generally relates to electronic systems and in particular it relates to gate drivers for Buck mode converters.
In conventional prior art buck mode (switch mode) converters employing MOS transistors as the high side and low side power switches, the high side switch is either a PMOS or NMOS. For a given design of buck mode converter, operating at some output voltage Vout, suppose that the input voltage drops. As the input voltage drops, the buck converter increases the duty cycle (D) (here, duty cycle D is defined as the on-time of the high side transistor multiplied by the frequency of switching of the high side transistor) Ultimately, there is some limitation on how large D can be. In the case of a buck converter where the high side transistor is a PMOS, the duty cycle can be increased all the way to 100%, since it is sufficient to simply pull the gate of the PMOS down to ground. However, PMOS output devices are larger than NMOS devices of comparable performance, so it is more expensive to use PMOS devices.
If an NMOS device is used, a method has to be found to drive the gate to a higher voltage than the source. In the prior art device shown in FIG. 1, this is done using a xe2x80x9cbootstrapxe2x80x9d capacitor and a diode or synchronous diode. The prior art circuit of FIG. 1 includes high side driver 20; low side driver 22; high side switch 24 (transistor); low side switch 26 (transistor); diode 28; bootstrap capacitor 30; inductor 32; output capacitance 34; output load resistance 36; and input voltage Vin. The disadvantage of this prior art system is that the bootstrap capacitor 30 depends on the converter maintaining less than 100% duty-cycle in order to recharge the bootstrap capacitor 30. Consequently, it is not possible to run the part down to as low an input supply as would be possible with 100% duty cycle.
A high power charge pump is used in some prior art devices to supply the entire high side gate-drive whether the device is running 100% duty cycle or not. However, the high power charge pump is very expensive in silicon area and uses several additional external capacitors. This is more costly in the switching converter IC and externally in the system.
In another prior art solution, an additional supply rail is used to power the NMOS drive. However, an additional rail is not available in many cases.
A switch mode converter uses the bootstrap capacitor to operate all the way to 100% duty cycle by adding only a very small amount of low-area extra circuitry. The additional circuitry includes a charge pump and a duty cycle detect device. When the duty cycle detect device detects that the converter is attempting to operate in 100% duty cycle, the charge pump provides additional charge to the bootstrap capacitor to ensure that the 100% duty cycle is maintained. Performance into dropout (when the input supply is actually lower than the desired output) is improved. A significant advantage in some applications (notably battery operation) is provided.